31. SETKÁNÍ KATEDER MECHANIKY TEKUTIN A TERMOMECHANIKY
26. – 28. června 2012, Mikulov
Comparison of sieve analysis and laser diffraction for size distribution of fine ash particles
Michaela Zárybnická1, Jiří Pospíšil1, Michal Špiláček1,
1Brno University of Technology, Faculty of Mechanical Engineering, Energy Institute,Technická 2896/2, Brno, , ,
Abstract This paper is discussing possible ways of how to determine the size distribution of fine solid particles,focusing on two methods in particular. The first is the sieve analysis which is one of the oldest methods suitable for sorting of groups of particles by size. The second is a modern method utilizing laser diffraction on small particles. For the measurement itself were used the sieve analyzer Analysette 3 PRO and universal laser diffraction device Analysette 22 MicroTec plus, both from the Fritch company. The comparison was made by analyzing fine ash particles from biomass burners. The particles were in the size range of 0.08 µm to 2000 µm.
1Introduction
Knowing the size of particles is important in many industries and science fields, such as material science, medicine, biology, power industry etc. The size of a particle is considered as the diameter (radius) of perfectly spherical particle. For any other shape of a particle the size parameter is her length that must be defined accordingly to the used method of measurement.
2Compared methods
Sieve analysis is based on utilization of a set of sieves with the known size of mesh. It is used mainly for separating the coarse particles from the fine ones. One of the advantages of this method is the known size of particles on particular sieves. The main disadvantage is the time demand and a possible destruction of a granularity of tough and fragile particles. In the sieving process, these particles tend to reduce their size dramatically due to abrasion and collisions which leads to inaccurate results that do not reflect the real size distribution of particles in the measured sample.
Laser diffraction is the present most widespread method used for the particle size measurement. The utilized physical principle is known from the begging of the 20th century, but only after the invention of suitable laser devices and computers have this method been developed. In the present day this method due to its flexibility and quickness is superseding other methods of the particle size measurement.
3Principle of measurement
The sieve analysis uses a set of sieves placed on top of each other, each with a different size of mesh that is decreasing in the direction of the gravitation gradient. For the analysis itself standardized round sieves with a rectangular mesh made of metal fibers. These sieves are usually used for analyzing powder samples with particle size in range from 40μm to 4mm. In our case the dry method of sieving was used on the particular device ANALYSETTE 3 PRO (fig.1). It is a typical device of which performance can be recognized by bare eyes. The construction is utilizing a cradle to which the sieves are fixed and which is placed on propulsion on which amplitude of vibrations, sieving intervals and a sieving time can be adjusted.
When the sieving process ends there is a fraction of the original sample on each sieve. Each fraction has particles with the size range that is in match witch the mesh size of the current and previous sieve. These fractions are then weighted. The results are weights of particles with particular size ranges which are one of the biggest advantages of this method.
Fig. 1Sieving analyzer Fig. 2 Laser analyzer Fig. 3 Concept of the laser beam shielding
The ANALYSETTE 22 MicroTec plus (fig. 2) is a universal analyzing device for particle measurement of suspensions, emulsions and solid matters that is utilizing laser diffraction. The device is composed of central measuring unit and dispersion module. In the central measuring unit two semiconductor lasers, each with the output of 7mW and wavelength of 532nm and 940nm respectively, are present. The measuring range is from 0,008µm up to 2000µmother dispersion unit is an ultrasound water (for a short term organic fluids or saturated inorganic salt solutions can be used) bath with the output of maximum of 50W and frequency of 40kHz. Any fluid gets in contact with only chemically stable materials. The entire device is operated via computer with MaScontrol software.Laser diffraction utilizes laser light that after it hits a particle is reflected in different than former direction as diffracted light. The amount of how much the laser beam is diffracted and how it is reflected depends on the size and optical attributes of each particle the beam hits. The diffracted light then hits Fourier’s len in which by a sensor (photodiode) the distribution of intensity of the diffracted light in the focal plane in dependence of incoming angle is measured. From results of the sensor are then calculated sizes of particles and their distribution. The concentration of particles inserted in the device must be low enough to avoid multiple laser light diffractions. On the contrary, the concentration must be high enough, so the particles are able to diffract enough lasers light for the sensor to detect. The optimum shielding of the laser beam for wet dispersion is 10 – 15 %. After the optimum amount of sample is inserted in the device, the measuring will start automatically. On fig.3 we can see the principle of laser beam shielding. In the upper section the laser beam is not shielded at all and its full intensity is hitting the photodiode. In the lower section a sample is introduced and the intensity that was hitting the photodiode has decreased due to the aforesaid shielding of laser beam caused by the introduced particles. For an optimal evaluation of the gathered data the device is using the Mie solution to Maxwell’s equations for fine particles. For particles many times bigger than the wavelength of laser beam the Fraunhofer approximation is used.
4Experiments and results
For the experiments fractions of ash from biomass burners were used. The first sample of ash (P1) originated from Dakon Damat Pyro 20G that is burning pieces of wood (spruce, pine tree). The sample has been taken from ash collector. The second sample (P2) originated from Slokov Variant SL 22D that is burning walnut tree chips and was taken from ashtray.
4.1Sieving analyzer
First, both samples were sieved in order to remove coarse particles. Multiple sieving courses have been done, each with bigger amplitude of vibrations and longer sieving intervals and a sieving time. The ash sample P1 have been sieved three times (samples 1, 2, and 3) and the ash sample P2 have been sieved two times (samples 4, 5). For the analysis nine sieves with different mesh sizes were used. Meshes with a size of 2mm and 1mm were used to remove the coarsest particles. After this, the samples ran through the meshes with sizes of (in given order) 500 µm, 200µm, 160µm, 125µm, 90µm, 63µm and 45µm. The fractions that were left on meshes below 500 µm (included) were then weighted and used for thorough analysis in the laser analyzer.
4.2Laser analyzer
Laser analyzer is showing the results in date sheets where for the corresponding range of particle sizes a frequency of such range to the whole sample is shown. All results are transparently arranged in form of a diagram.
Diagram 1 Ash P1, sample 2, 45µm Diagram 2 Ash P1, sample 1, 200µm
From the diagram 1 that is showing results obtained from the analysis of the P1 residue on 45µm mesh we can clearly see that there are many particles that are smaller than 45µm (areaA).Logically, there should be only particles of the size of 45µm and greater. On the diagram 2 we can see a situation when on the 200 µm mesh, according to the laser analysis, there are no particles bigger than 200µm. All the particles have been identified as smaller than 200 µm (included). Similar results have been obtained from the majority of the measurements.
In table 1 are shown the results of the executed analyses from which by the laser diffraction there were detected particles bigger than the size of the mesh they were taken from (see diagram 1, area B). Evaluation of the obtained results was made with these assumptions: all particles are flawlessly spherical and have a constant density ρ=1200kg/m3.
From the date sheets were obtained the frequencies A [%] (particles smaller than the mesh size of corresponding sieve) and B [%] (particles bigger than the mesh size of corresponding sieve). Values of these frequencies were transformed into weight portions (C, D [%]). Obtained results are lucidly arranged in table 1 and figure 3.4.
Tab. 1Non-zero frequencies and weight portions of evaluated samples
Diagram 3 Frequency of particles bigger Diagram 4 Weight portions of particles bigger than the than the mesh size mesh size of corresponding sieve
of corresponding sieve
4Conclusion
Evaluation of the analyzed ash samples shows a high amount of fine particles in individual samples. Transformation of frequencies into the weight portions shows a significant impact of a relatively minor number of bigger particles on the results obtained from sieving analysis. Evaluated samples shows a significant cohesiveness of ash particles that is causing overvaluation of the results obtained by sieving analysis from the sieves with the used mesh size. Executed trials did do not show any improvements in stated overvaluation by using more intense and repeated sieving. With the cohesiveness of the ash particles in consideration, the most significant boon of the laser analysis is the high-quality dispergation of the sample in water bath that prevents the flocculation of the ash particles and leads to more accurate evaluation of samples of ash particulates.
Acknowledgement The present work has been supported by European Regional Development Fund in the framework of the research project NETME Centre under the Operational Programme Research and Development for Innovation.
Sources
[1]PABST, W., GREGOROVÁ, E. Charakterizace částic a částicových soustav. Praha: Vysoká škola chemicko technologická, 2007. s. 1-22
[2]Firemní podklady firmy Fritsch kpřístroji „ANALYSETTE 22“ MicroTec plus